The small and rocky planet Mercury is the closest planet to the Sun; it speeds around the Sun in a wildly elliptical (non-circular) orbit that takes it as close as 47 million km and as far as 70 million km from the Sun. Mercury completes a trip around the Sun every 88 days, speeding through space at nearly 50 km per second, faster than any other planet.
Because it is so close to the Sun, temperatures on its surface can reach a scorching 467 degrees Celsius. But because the planet has hardly any atmosphere to keep it warm, nighttime temperatures can drop to a frigid -183 degrees Celsius.
Because Mercury is so close to the Sun, it is hard to see from Earth except during twilight. Until 1965, scientists thought that the same side of Mercury always faced the Sun. Then, astronomers discovered that Mercury completes three rotations for every two orbits around the Sun. If you wanted to stay up for a Mercury day, you'd have to stay up for 176 Earth days.
Like our Moon, Mercury has almost no atmosphere. What little atmosphere exists is made up of atoms blasted off its surface by the solar wind and has less than a million-billionths the pressure of Earth's atmosphere at sea level. It is composed chiefly of oxygen, sodium, and helium. Because of Mercury's extreme surface temperature, these atoms quickly escape into space and are constantly replenished. With no atmosphere to protect the surface, there has been no erosion from wind or water, and meteorites do not burn up due to friction as they do in other planetary atmospheres. Mercury's surface very much resembles Earth's Moon, scarred by thousands of impact craters resulting from collisions with meteors. While there are areas of smooth terrain, there are also cliffs, some soaring up to a mile high, formed by ancient impacts.
The Caloris Basin, one of the largest features on Mercury, is about 1,300 km in diameter. It was the result of an asteroid impact on the planet's surface early in the solar system's history, the probable cause of the strange surfaces on the opposite side of the planet. Over the next half-billion years, Mercury actually shrank in radius from 2 to 4 km as the planet cooled from its formation. The outer crust, called the lithosphere, was compressed and grew strong enough to prevent the planet's magma from reaching the surface, effectively ending the planet's period of geologic activity. Evidence of Mercury's active past is seen in the smooth plains in the Caloris basin.
Mercury is the second smallest planet in the solar system, larger only than Pluto, the most distant planet in our solar system. If Earth were the size of a baseball, Mercury would be the size of a golf ball. Viewed from Mercury, the Sun would look almost three times as large as it does from Earth. Mercury is the second densest body in the solar system after Earth, with an interior made of a large iron core with a radius of 1,800 to 1,900 km, nearly 75 percent of the planet's diameter and nearly the size of Earth's Moon. Mercury's outer shell, comparable to Earth's outer shell (called the mantle) is only 500 to 600 km thick.
Only one spacecraft has ever visited Mercury: Mariner 10 in 1974-75.
Mariner 10's discovery that Mercury has a very weak magnetic field, similar to but weaker than Earth's, was a major surprise. In 1991, astronomers using radar observations showed that Mercury may have water ice at its north and south poles. The ice exists inside deep craters. The floors of these craters remain in perpetual shadow, so the Sun cannot melt the ice.
Mercury: Facts & Figures
Discovered By: Known by the Ancients
Date of Discovery: Unknown
Average Distance from the Sun
Metric: 57,909,175 km
English: 35,983,095 miles
Scientific Notation: 5.7909175 x 107 km (0.38709893 A.U.)
By Comparison: Earth is 1 A.U. (Astronomical Unit) from the Sun.
Metric: 46,000,000 km
English: 28,580,000 miles
Scientific Notation: 4.600 x 107 km (0.3075 A.U.)
By Comparison: 0.313 x Earth
Metric: 69,820,000 km
English: 43,380,000 miles
Scientific Notation: 6.982 x 107 km (0.4667 A.U.)
By Comparison: 0.459 x Earth
Metric: 2,439.7 km
English: 1,516.0 miles
Scientific Notation: 2.4397 x 103 km
By Comparison: 0.3825 x Earth
Metric: 15,329.1 km
English: 9,525.1 miles
Scientific Notation: 1.53291 x 104 km
Metric: 60,827,200,000 km3
English: 14,593,200,000 mi3
Scientific Notation: 6.08272 x 1010 km3
By Comparison: 0.054 x Earth's
Metric: 330,220,000,000,000,000,000,000 kg
Scientific Notation: 3.3022 x 1023 kg
By Comparison: 0.055 x Earth's
Metric: 5.427 g/cm3
By Comparison: 0.984 x Earth
Metric: 74,800,000 km2
English: 28,900,000 square miles
Scientific Notation: 7.48 x 107 km2
By Comparison: 0.108 x Earth
Equatorial Surface Gravity
Metric: 3.7 m/s2
English: 12.1 ft/s2
By Comparison: If you weigh 100 pounds on Earth, you would weigh 38 pounds on Mercury.
Metric: 15,300 km/h
English: 9,500 mph
Scientific Notation: 4.25 x 103 m/s
By Comparison: Escape Velocity of Earth is 25,022 mph
Sidereal Rotation Period (Length of Day)
58.646 Earth days
By Comparison: 58.81 x Earth
Sidereal Orbit Period (Length of Year)
0.241 Earth years
87.97 Earth days
By Comparison: 0.241 x Earth
Mean Orbit Velocity
Metric: 172,341 km/h
English: 107,088 mph
Scientific Notation: 47,872.5 m/s
By Comparison: 1.61 x Earth
By Comparison: 12.3 x Earth
Orbital Inclination to Ecliptic
Equatorial Inclination to Orbit
By Comparison: Earth's equatorial inclination to orbit is 23.45 degrees.
Metric: 356,000,000 km
English: 221,000,000 miles
Scientific Notation: 3.56 x 108 km
By Comparison: 0.385 x Earth
Minimum/Maximum Surface Temperature
Metric: -173/427 °C
English: -279/801 °F
Scientific Notation: 100/700 K
By Comparison: Earth's temperature range is ~ 185/331 K.
By Comparison: Earth's atmosphere consists mostly of N2, O2